1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile and the like.
2. Related Background Art
In a conventional image forming apparatus as shown in FIG. 5, a toner image formed on a drum-shaped electrophotographic photosensitive member (referred to as "photosensitive drum" hereinafter) 101 is firstly-transferred onto an intermediate transfer member 105 once and then the toner image transferred to the intermediate transfer member is secondary-transferred onto a transfer material P by a secondary transfer roller 108, thereby forming an image. This image forming apparatus is effective as a color image forming apparatus or a multi-color image forming apparatus in which a plurality of color component images of color image information or multi-color image information are successively transferred in a superimposed fashion to reproduce and output a color image or a multi-color image, or an image forming apparatus having a color image forming function or a multi-color image forming function.
In the image forming apparatus capable of forming the color image, conventionally, the peripheral rotation speed of the photosensitive drum 1 is selected to be same as that of the intermediate transfer member 105, and the peripheral rotation speed of the intermediate transfer member 105 is selected to be equal to those of the secondary transfer roller 108, a pair of regist rollers 109, and a pair of fixing rollers 110 of a fixing device. The intermediate transfer member 105 is urged against the photosensitive drum 101 with predetermined pressure to form a nip (first transfer station) 106a at which the plurality of color toner images formed on the photosensitive drum 101 are successively firstly-transferred onto the intermediate transfer member 105 in the superimposed fashion. The secondary transfer roller 108 is disengaged from the intermediate transfer member 105 until the plurality of toner images are superimposed on the intermediate transfer member 105, and is urged against the intermediate transfer member 105 at a timing for collectively transferring the plural color toner images are transferred onto the transfer material P.
Further, the pair of regist rollers 109 is disposed at an upstream side of the secondary transfer roller 108 in a conveying direction for the transfer material P and cooperates with the intermediate transfer member 105 to convey the transfer material P at a predetermined timing so that the plural color toner images on the intermediate transfer member 105 are secondary-transferred onto a predetermined position on the transfer material P at a secondary transfer nip 106b. The pair of fixing rollers 110 of the fixing device are disposed at a downstream side of the secondary transfer roller in the transfer material conveying direction so that non-fixed color toner images on the transfer material P are fixed to the transfer material P with predetermined pressure and heat.
By the way, in the above-mentioned conventional image forming apparatus, since the photosensitive drum 101 and the intermediate transfer member 105 are statically pressurized against each other at the nip (first transfer nip) 106a between the photosensitive drum 101 and the intermediate transfer member 105, when each color toner image on the photosensitive drum 101 is transferred onto the intermediate transfer member 105, poor transferring (void) may occur at a central portion of the toner image. For example, as shown in FIG. 6, in case of alphabet "A", edges of the character may be emphasized due to the void to form a hollow character.
Further, if the peripheral speed of the intermediate transfer member 105, secondary transfer roller 108, pair of regist rollers 109 or fixing rollers 110 of the fixing device is differentiated from the other peripheral speeds, the conveying speeds for the transfer material P will not become constant between the units. As a result, the image quality is worsened due to transfer deviation or the image is not formed on the predetermined position on the transfer material P or tip and/or trail ends of the toner image are expanded or contracted, thereby worsening positioning accuracy for forming the toner image.
Further, if the shifting speed of the transfer material P being passed through the secondary transfer nip 106b becomes slower than the peripheral speed of the intermediate transfer member 105, the intermediate transfer member will be braked. Further, if the transfer material conveying speed of the pair of regist rollers 109 becomes slower than the peripheral speed of the intermediate transfer member 105, the intermediate transfer member 105 is pulled by the transfer material P to brake the intermediate transfer member 105. As a result, in an arrangement in which a circumferential distance from the first transfer nip 106a to the secondary transfer nip 106b in a rotational direction of the intermediate transfer member 105 is smaller than a length of the toner image to be formed, when the secondary transferring of the tip end portion of the last color toner image is started while such toner image is being firstly-transferred, or when the firstly transferring of a next first color toner image is started during the secondary transferring of the preceding toner image, there arises rotational unevenness of the intermediate transfer member 105 or urging force between the intermediate transfer member 105 and the photosensitive drum 101 at the first transfer nip 106a is changed.
For this reason, for example, when a full-color image is formed, there arises positional deviation between the first to third color toner images firstly-transferred to the intermediate transfer member 105 and fourth color (last color) toner image or positional deviation (color deviation) between the next first color toner image to be firstly-transferred and the second to fourth color toner images.